1. Field of the Invention
This invention relates to a solar water heating system, and more particularly to a solar water heating system which has a better heating efficiency and a longer service life.
2. Description of the Related Art
Solar water heating for domestic, commercial, or industrial purposes is one of the oldest and most successful applications of solar-thermal technology. The most widely used solar water heating system, and one that is suitable for use in relatively warm climates where freezing is a minor problem, is the thermosyphon type.
Referring to FIG. 1, a conventional solar water heating system is shown to comprise a water storage tank 10 which is provided with a hot water outlet pipe 100 and a cold water inlet pipe 102, a solar-collector unit 12 for collecting solar energy and for heating the water therein, a downcomer pipe 14 for transferring cold water from the water storage tank 10 to the solar-collector unit 12, and a return pipe 16 for transferring hot water from the solar-collector unit 12 to the water storage tank 10.
The hot water outlet pipe 100 and the cold water inlet pipe 102 are connected respectively to upper and lower portions of two opposite ends of the water storage tank 10. The downcomer pipe 14 and return pipe 16 are connected to lower and upper portions of the two opposite ends of the water storage tank 10. When there is no hot water running out of the water storage tank 10 from the hot water outlet pipe 100, the water storage tank 10 is in a closed state and therefore, cold water will not flow into the water storage tank 10 through the cold water inlet pipe 102.
The solar-collector unit 12 is a flat-plate type collector unit that is located below the water storage tank 10 and that is supported on the ground at an angle, as best illustrated in FIG. 2, in order to absorb incoming radiant energy and convert the same into heat. The downcomer pipe 14 is connected to a lower portion of the solar-collector unit 12, while the return pipe 16 is connected to an upper portion of the solar-collector unit 12. The solar-collector unit 12 has a plurality of interconnecting, parallel, water tubes 18 where the water is to be heated by solar radiation. The system is filled with water, and when the sun shines on the solar-collector unit 12, the water in the water tubes 18 is heated and becomes less dense than the water in the downcomer pipe 14. The heated water rises by thermosyphon action from the water tubes 18 through the return pipe 16 into the water storage tank 10, and is replaced by cold water from the cold water inlet pipe 102 through the downcomer pipe 14. This action continues as long as the sun shines on the solar-collector unit 12 with adequate intensity. In this way, the water temperature in the water storage tank 10 will continue to rise.
In addition, the water used in the system usually contains minerals which causes the formation of the fouling on the internal face of the water tubes 18 when the water is heated. The amount of fouling will gradually increase and forms a heat insulation layer which adversely affects heat transfer between the water and the water tubes 18. In addition, since the water is circulated in the system by the thermosyphon action, the diameter of the water tubes 18 must not be too large. Therefore, the water tubes 18 are likely to be blocked by the fouling after being used for a period of time, thus resulting in an unduly short service life.